This invention relates to a new class of catalysts and to their production. More particularly, this invention relates to catalysts containing silver and transition metal moieties. These silver/transition metal catalysts are useful catalysts for isomerization, dehydrogenation, reduction and mild oxidations.
The use of silver alone as a catalyst, or promoted by certain other metals such as gold, copper, iron and manganese in the oxidation of ethylene to ethylene oxide, however, is well-known (P. G. Ashmore, Catalysis and Inhibition of Chemical Reactions, Butterworths, London 242 (1963)) and has been taught by many examples in the prior art. For example, U.S. Pat. No. 2,605,239 teaches a silver-beryllium oxide catalyst for oxidizing ethylene to ethylene oxide. The patent teaches the source of the silver as being a reducible oxygen-containing compound such as the silver salt of a carboxylic acid, i.e., formic, acetic, propionic, oxalic, malonic and maleic acids, as well as others. Suitable promoters indicated include metals such as copper, aluminum, manganese, cobalt, iron, nickel, gold, cadmium and zinc. These may be incorporated by mixing or by coprecipitation.
It is also widely known that the addition of a second component to a material which acts as a catalyst as a single component may result in a combination having increased catalytic activity. It is equally widely known that the properties of a catalyst are determined by the total history of its preparation. Many catalysts consist of a major component of high area which is often referred to as the "support" with one or more components of smaller percentage. Even with high-area catalysts it is well-known that only a small percentage of the surface is active. The effect of the support on the other catalyst components and indeed the chemistry of surfaces and complexes is not well-understood. Accordingly, it is difficult to draw a distinction between the catalytic function of the "support" and the other catalyst components.
The use of silver in my invention in conjunction with transition metals of the Periodic Table results in unique catalysts whose specific catalytic activity can only be gauged by actual performance tests. The exact function of the silver which is both a support and catalyst component is not understood, i.e., the silver may be acting as the catalyst and the transition metals may be acting as "promoters". However, it is not feasible to differentiate in general between the two as the function between the two can vary for individual reactions.
It is, therefore, a general object of my invention to provide a new class of catalysts with high area silver supports which incorporate transition metals as co-catalysts and as promoters. A more particular object is to provide a practical and economic process for the manufacture of these catalysts. Another object of my invention is to provide catalysts with unique performance characteristics when employed in dehydrogenation, reduction and mild oxidation reactions. A further object of my invention is to provide improved silver catalysts and processes for using them in specific reactions which are of particular utility such as the direct vapor phase dehydrogenation of ethylbenzene to styrene. The nature of still other objects of my invention will be apparent from a consideration of the descriptive portion to follow.
It is my discovery that the above and other objects of the invention are attained by the silver/transition metal catalysts herein described.
The poly silver salts decarboxylate under the process conditions of my invention and form high polymers with the silver metal developing the desired high surface area as a characteristic property. It is important that polycarboxylic acid silver salts be used in my invention because, although the monocarboxylic acid silver salt decarboxylates under the process conditions of my invention, the monosilver salt residue forms dimers and low polymers with the silver metal in the form of a globule with low surface area and little catalytic activity. It is important that polycarboxylic acid transition metal salts be used to avoid interaction between salts that could lead to silver monocarboxylates. It is also preferable that the transition metal salts and silver salts be derived from the identical polycarboxylic acids rather than different polycarboxylic acids if the coprecipitation method of preparation is used. Use of other polycarboxylic acids in such circumstances can result in lessened catalytic activity because of different decomposition temperatures and the possible chemical interaction between the salts before the decomposition temperatures are reached. Coprecipitation might not occur with the two salts, preventing an ultimate intimate mixture of the two metals. If the metal salts are mixed intimately, as by milling together, the silver and transition metal salts may be derived from different polycarboxylic acids as, for example, disilver isophthalate and cobalt (II) trimestate.
The silver/transition metal catalysts prepared by the method of calcining silver and transition metal salts followed by oxidation at controlled temperatures quite suprisingly can act as catalysts for controlled mild oxidation, dehydrogenation and isomerization reactions at temperatures from 130.degree. to 500.degree.C. The lower limit of 130.degree.C. is the lowest reasonable temperature consistent with a reasonable speed of reaction. The upper limit of 500.degree.C. is the highest reasonable temperature consistent with physical characteristics of the reactants such as melting points, boiling points and decomposition temperatures.
For example, as will be shown in the illustrative examples, the silver/transition metal catalyst (where cobalt and nickel are transition metals) quite surprisingly dehydrogenates ethylbenzene to styrene with good to excellent selectivity. Generally the major constituent of catalysts for dehydrogenating ethylbenzene to styrene is iron oxide, although other oxides can be used such as those of magnesium, chromium, cesium, tungsten with, in some instances, oxides of aluminum, silicon and zinc. In these examples, conditions of reaction to dehydrogenate ethylbenzene to styrene are one atmosphere pressure with the temperature within the range from 220.degree. to 300.degree.C. The preferred temperature with the silver/nickel catalyst is 260.degree.C. With the silver/cobalt catalyst, the preferred temperature is 250.degree.C.
For example, as will be shown in the illustrative examples, the silver/transition metal catalysts (where nickel, platinum or palladium comprise the transition metals), quite surprisingly can effect the oxidation of one methyl substituent on a benzene ring without affecting another alkyl or halogen substituent on the same ring. This system, which can be described as one of mild oxidation, can oxidize the individual isomeric xylenes to their corresponding m- and p-toluic acid, methyl benzaldehydes and methyl benzyl alcohols. This system of mild oxidation quite surprisingly attacks selectively the para-methyl substituents in preference to alkyl groups of more than one carbon in other positions and halogen substituents.
In the mild oxidation of meta- and paraxylenes using silver/nickel/platinum/or palladium catalysts, the conditions of reaction can be relatively mild. The range of temperatures can be from 130.degree.C. at one atmosphere pressure to 200.degree.C. at ten atmospheres pressure. The preferred conditions are within the range from 136.degree.C. to 142.degree.C. at one atmosphere pressure. In the case of the halogenated toluenes with one to three halogens attached to the ring, i.e., fluorine, chlorine and bromine, but not including iodine, the conditions of reaction are more stringent to give the correspondingly halogenated benzaldehydes and benzyl alcohols. A silver/palladium catalyst is used. The range of temperature can be from 140.degree.C. at one atmosphere pressure to 200.degree.C. at one to five atmospheres pressure. The preferred conditions are 155.degree.-165.degree.C. at one atmosphere.
In the mild oxidation of the methyl group of para-tert-butyltoluene to the corresponding acid, aldehyde, alcohol and ester, the range of temperature using a silver/palladium catalyst can be from 160.degree.C. at one atmosphere to 250.degree.C. at seven atmospheres. Preferred conditions are from 188.degree. to 191.degree.C. at one atmosphere. A similar oxidation of 1-methylnaphthalene using silver/palladium catalyst can be from 200.degree.C. at one atmosphere to 300.degree.C. at seven atmospheres. Preferred conditions are from 241.degree.C. to 245.degree.C. at one atmosphere.
For example, the silver/transition metal catalyst (where palladium comprises the transition metal) quite surprisingly can effect the reduction of nitrobenzene to azobenzene with excellent selectivity. Prior art teaches the treatment of nitrobenzene with caustic soda in the presence of glucose, or with zinc dust and sodium hydroxide in methanol, or with iron and sodium hydroxide, or with hydrogen and bismuth as a catalyst. In this reduction, the range of temperature can be from 150.degree.C. at one atmosphere to 250.degree.C. at seven atmospheres. The preferred condition is the reflux temperature of nitrobenzene at one atmosphere, 211.degree.C.
For example, the silver/transition metal catalyst (where vanadium comprises the transition metal) quite surprisingly can effect the isomerization of 1-pentene to the cis and trans forms of 2-pentene as well as to 2-methylbutene-2 at temperatures from 360.degree. to 450.degree.C. at one atmosphere. Preferred temperature is 400.degree.C.
For example, the silver/transition metal catalyst (where vanadium comprises the transition metal) quite surprisingly can effect the oxidation of toluene to benzaldehyde with excellent selectivity at temperatures from 400.degree. to 500.degree.C. Preferred temperature is 450.degree.C. at one atmosphere.
For example, the silver/transition metal catalyst (where palladium comprises the transition metal) quite surprisingly can effect the oxidative carboxymethylation of para-tert-butyltoluene to 2-methyl-5-tert-butylphenyl acetic acid with excellent selectivity. The range of temperatures is from 100.degree.C. at one atmosphere to 150.degree.C. at seven atmospheres. Preferred temperature is 130.degree.C. at one atmosphere.